Historically, aircraft electrical generating systems have involved electrical generating and distribution systems that provided alternating current at a constant frequency. In fact, most of the aircraft in the world today are provided with constant speed drives that are coupled to generators to furnish constant frequency alternating current to power the numerous AC powered equipment of the aircraft. The engines of some of these aircraft are typically started by the application of pneumatic or AC power to the generator to drive the same as motors during aircraft engine startup.
There has developed a need especially in military aircraft for the generation of DC power for such equipment as radar and avionics. In fact, in this new generation of aircraft, a very high proportion of the power required must be of a DC nature while the need for AC is relatively small.
Typical of the AC type starter-drive systems of the prior art is that shown and described in the U.S. Pat. Nos. to R. W. Reynolds et al 3,274,855 and Aleem 3,786,696. The Reynolds et al and Aleem arrangements each includes a hydraulic unit with a mechanical differential coupled to an aircraft engine and an alternator, which alternator is normally driven by the engine while the aircraft is in operation. The hydraulic unit with the mechanical differential transmits power frpom the engine to the generator to drive the latter at a constant speed regardless of variations in engine speed or load. The alternator is also employed for transmitting power from the generator to the engine when the generator is operated as a motor for purposes of starting the engine. Neither Reynolds et al nor Aleem are capable of providing DC power combined with a DC powered start. The invention to be described hereinafter provides a fresh departure from these typical prior art approaches of Reynolds et al and Aleem.